A 28-year-old man presents with haematuria, mild hypertension, and normal renal function 2 weeks after a streptococcal pharyngitis. Serum complement C3 levels are low. A 35-year-old woman with a 10-year history of systemic lupus erythematosus (SLE) presents with haematuria, proteinuria, and hypertension. Anti-dsDNA antibodies are positive and serum C3 is also low. Which single feature best distinguishes post-streptococcal glomerulonephritis (PSGN) from lupus nephritis?